Multiplex receiving system



Mafch 21, 1939.

W. L. CARLSON ET AL MULTIPLEX RECEIVING SYSTEM Filed Sept. 2.4, 1936 2 Sheets-Sheet 1 WDWSQW Rx 8.63m

Patented Mar. 21, 1939 UNITED STATES PATENT OFFICE MULTIPLEX RECEIVING SYSTEM Application September 24, 1936, Serial No. 102,286

2 Claims.

Our invention relates to a multiplex radio receiving system. More particularly our invention includes in combination a plurality of antennas, band amplifiers, and distributing system for mul- 5 tiplex radio reception.

We are aware of systems in which a plurality of radio receivers are connected to a common antenna. Such systems are often characterized by mutual reactions when the several receivers are tuned, or cross talk if coupling tubes are connected between the several receivers and the common antenna. We are further aware that a common antenna has been used with a broadly tuned amplifier whose'output circuit in- 5 cludes the input to a plurality of radio receivers.

This arrangement requires a high-gain amplifier which has a uniform amplification characteristic over a very broad band of frequencies, if the system is to be used with a plurality of all 20 wave receivers. An amplifier of high gain and uniform amplification is usually subject to cross talk, interference, and hiss when directly coupled to an antenna and to further difiiculties due to the resonant characteristic of the antenna.

25 We propose to overcome these diificulties by connecting a plurality of antennas, each resppnsive to radio signals covering a band of frequencies, to a plurality of radio frequency amplifiers, each substantially uniformly responsive 30 to" currents impressed on it by its associated antenna. Suitable resonant networks, connected between the antennas and the amplifiers, may be used to attenuate interfering signals which might otherwise cause cross talk or block the ampli- One of the objects of our invention is to provide means whereby a plurality of all wave receivers may be operated from common antennas and amplifying systems.

40 Another object is to provide a plurality of antennas, each responsive to currents of a band of radio frequencies, and means coupling the antennas to band responsive amplifiers whose output circuits have a common connection to a plu- 45 rality of all wave receivers.

A further object is to provide means for attenuating interfering signals which tend to block the amplifiers which couple a plurality of receiving antennas and a plurality of all wave radio receivers.

Additional objects are: to obtain highgain from the antenna to the input of the first amplifier to reduce amplifier hiss; to limit cross talk or interference to a narrow band by the use of a plui5 ,rality of band amplifiers; to keep the amplifier tubes within their rated capacity to lessen cross modulation; and to attenuate harmonic output currents by employing narrow band output transformers.

In the accompanying drawings Fig. 1 is a schematic circuit diagram of one embodiment of our invention,

Fig. 2 is a graph showing the frequency characteristics of the radio frequency coupling transformers, and 10 Fig. 3 is a schematic illustration of one arrangement of a plurality of antennas.

While the foregoing objects are the prime desideratum, the number of channels formed by separate antennas and separate band amplifiers can be increased to an impractical extent. By way of example, we have found the following channels and channel widths, satisfactory for receiving signals whose frequency is between 2 megacycles and 18.1 megacycles.

First channel 3-4.6 m. 0.

Second channel 4.6-7.3 m. 0.

Third channel 73-10 m. 0.

Fourth channel 10-12.7 m. 0.

Fifth channel 12.7-15.4 111. c.

Sixth channel 15.4-18.1 m. 0. In the present broadcast frequency band we have employed a single antenna and three amplifiers to cover the following ranges:

Amplifier 1 530 to 900 kilocycles Amplifier 2 900 to 1250 kilocycles Amplifier 3 1250 to 1600 kilocycles Our invention may be best understood by reference to the accompanying circuit diagram Fig. 5 1, which schematically illustrates one embodiment. Referring to the circuit diagram, an antenna l is coupled through a suitable transmission line 3 to a transformer 5. The primary of the transformer 5 is a tuned circuit, which is comprised of a capacitor '1 and an inductor 9. Within the inductor winding 'is an adjustable magnetite core II. The inductor 9 is mutually coupled to a second inductor 13, which also includes an adjustable magnetite core 15. The second inductor I3 is connected to the grid circuit of an amplifier 17, which may be any suitable thermionic tube or the like.

The anode circuit of the amplifier tube 11 includes a transformer !9.' This transformer is comprised of a pair of mutually coupled inductors 2|, 23, which preferably include adjustable magnetite cores 25, 21. The primary inductor 2| is shunted by a resistor 29. The secondary inductor 23 is connected to the grid circuit of an amplifier 3| which may be of any suitable type. The output of the amplifier 3| includes a transformer 33. This transformer 33 is also composed of a primary inductor 35 and a mutually coupled secondary inductor 31. These inductors 35, 31, may preferably include adjustable magnetite cores 39, il The secondary circuit of the transformer 33 is completed by a capacitor 43 and a shielded line 45, which is connected to the. secondary circuit at an appropriate point. The shielded line is connected to a distribution network which will be described below.

Inasmuch as the second (4.6-7.3 m. c.) and third (7.3-10 m. c.) channels are composed of apparatus and circuits which differ from the first channel (2-4.6 m. 0.) only in the range of frequency response, no useful purpose would be served by repeating the foregoing description. It will be observed that the shielded line 45 is connected to the output circuits of each amplifier. Additional channels (IO-12.7 m. c.'; 12.7-15.4 m. c.; 15.4-18.1 m. 0.; etc.) not shown, may be similarly included in the system.

Having described the antennas and amplifiers for the high frequency channels, the broadcast channel will be considered. In the broadcast band, 530 to 1600 kilocycles, a single antenna 41 is connected through a transformer 49 to a transmission line 5| which terminates in several resonant circuits. A plurality of attenuating networks 53 may be connected in the transmission line to reduce cross talk and interference caused by signal currents of high amplitude. Each primary inductor 55 is mutually coupled to a resonant secondary circuit which includes an inductor 51 and a capacitor 59. Adjustable magnetite cores 6| are incorporated in the primary inductors 55.

The transmission line 5| terminates in a plurality of resonant circuits 53, 55, 6'1, which are connected in shunt. The inductors '69, 1|, 13,

' of these circuits are preferably tuned by magnetite cores 15, ll, 19.. These inductors 69, H, 13, are mutually coupled to secondary inductors 8|, 83, 85, which. include adjustable magnetite cores 81, 89, 9|. The secondary inductors 8|, 83, 85areconnected to thermionic amplifiers 93, 95, 91. The output circuit of these amplifiers in.- cludes resonant transformers 99, IM, Hi3, which are composed of primary inductors including magnetite cores and resonant secondary circuits. These secondary circuits I05, I01, I09 are each comprised of capacitors and inductors which preferably have adjustable magnetite cores. The secondary circuits Hi5, I01, m9 are connected to the'shielded output lead 45.

The combined outputs from the several separate channels may cover a very broad frequency band; forexample, 2 megacycles to 18 .1 mega.- cycles and 530 kilocycles to 1600 kilocycles. Precautions must be observed in the design of a distribution network to cover such frequency range. In such network we have found a bifilar-wound, step-down transformer with a two toone ratiosatisfactory. In the preferred arrangement one bifilar-wound transformer Ill feeds into four similar transformers H3. If one of these four transformers is omitted as shown, a resistor 114' of proper terminating value may be'substituted to maintain the desired impedance match. Each of the four transformers l3 in turnv is connected to four more transformers H5, which also have bifilar-two to one step-down windings. The secondary windings of these transformers H5 each terminate'in four. resistorslll; The input circults of the all-wave receivers are each connected to a separate one of these resistors I I1. One such connection is represented by theblock H9. In the arrangement illustrated provision has been made for sixty-four receivers. It should be understood that this distribution network may be multiplied to serve any required number of receivers. Our invention is not limited to this precis e distribution network as other arrangements may be employed. r

In the' foregoing description no mention has been made of the characteristics of the radiofrequency transformers 5, I9, 33, which couple the antennas to the amplifiers H, the amplifiers l1, 3| to each other, and the output of the amplifiers 3| to the distribution line 45. Each transformer consists of two tuned coupled circuits adjusted to give a slightly double peaked response of the proper band width. In each transformer one of the tuned circuits is made as low loss as possible. The selectivity factor of the other tuned circuit is adjusted to the proper value for the given band width. With the input and output,

transformers the damping is supplied bythe surge impedance of the line which acts as a. resistance in series with the tuned circuit. The band width of such a series circuit is a function of L/R (L=inductance, R=resistance). Since R is the same for each channel, L is also the same; The inductance of the coil in series with the transmission line is the same in all short-wave input transformers. The resonant frequency is varied by changing the series capacity.

In the interstage transformrsthe damping is supplied by a shunt resistor on the primary.- With a shunt resistor the band width is a function of RC (C=capacity). Since C is constant, R is the same in each channel to give the same band width. In each channel there are four tuned circuits associatedwith either the plate or. grid of a vacuum tube. In each of these circuits the tube capacity is the chief capacity of the cir-.

cuit, all other capacities being kept toa mini-' mum. This: is the condition for amaximum prod-.

uct of gain times band width.

All of the'resonant circuits of each channel are tuned to the proper frequency by the adjustable magnetite cores. An exceptionis'the 24.6mm.

channel, where the tight coupling'requiredmakes' it impossible to separately tune thetwo resonant circuits of each transformer. Fixed tuning is used in this band.

The inputtransformers have a gain of about five from the transmission line to the first'grid;

The interstage transformers have a gainof about. 8 from grid to grid. The output transformershave a gain of about unity from'grid to line." The overall gain from line to line is about 40. Figure 2 gives a performance curve'A on a typicalinput transformer. Curve-B represents an interstage transformer. A graph C shows the corresponding output transformer and the overall curve for the channel is designated as D. f i

Although the output transformers are all contransformers are used, however, the performance curves are radically changed when adjacent transformersare connected across the line. a

For the broadcast band the previously -mentioned considerations indicating; the-desirability:

If broader The inter of narrow channels apply with increased force,

' because of the greater utilization of transmission channels within these frequencies. A band width of about 350 k.c. has been found satisfactory. for this narrow band width the available gain per stage is sufficient to require only a single tubeper channel. The input transformers have a gain of about 20 and the outpout transformers a gain of about 2. The principles involved in the design of these transformers are the same as for those of the short-wave channels.

'WhiIe the antenna structures may follow any conventional design, we have found dipoles resonant to the center of each frequency band most suitable for the higher frequencies. If desired, the 2-4.6 m. 0. band may be served by a T antenna, which may be coupled through a suitable transformer (see reference character 49) to the transmission line. The dipole antennas may be erected parallel to each other with a spacing of two feet and in a vertical or a horizontal plane. One suitable arangement is to erect a pair of supportsapproximately eighty feet apart. The antenna wires are connected between these supports and are appropriately insulated to obtain the required effective lengths as follows: 4.6-7.3 m. 0.: 80 feet, 73-10 m. c.=50 feet; 10-12.? m. c.=40 feet; 12.7-15.4 m. c.=30 feet and 15.4-48.1 m. 0.: 26 feet. The supporting wires on the smallest dipole (26 feet) may be connected to the broadcast antenna to increase its capacity. Likewise the supporting wires for the dipole of a length of 40 feet may be used to increase the capacity of the antenna used for 24.6 m. c. reception. The antennas responsive to adjacent frequency bands are preferably spaced asfar from each other as possible. The foregoing antenna arrangement is illustrated in Fig. 3.

Thus we have described a plurality of antennas each responsive to frequencies within preassigned channels or frequency bands. Each such antenna isconnected to an amplifier which is substantially uniformly responsive to frequencies within the I respective channels. Thebroadcast band antenna is connected to a transmission line within which attenuating networks may be included to reduce interference from signal currents of great amplitude. The outputsrof the several amplifiers are connected to a common distributing network to which a plurality of all wave radio. receivers may be connected.

We claim:

1. In a system of the character described a plurality of antennas each responsive to currents of substantially different high frequencies, a plurality of high frequency amplifiers each responsive to different bands of frequencies of the order of 2.5 megacycles wide and including one of said antenna responsive frequencies, means coupling said antennas and amplifiers having similar responsive frequencies, an antenna responsive to currents of the broadcast frequency range, a plurality of amplifiers each of which are responsive to different bands within said range of broadcast frequencies, means coupling said last-mentioned antenna and said last-mentioned amplifiers, a distributing network effectively connected to the output of said first and second mentioned amplifiers, said distributing network including a single step down transformer and four similar transformers connected to the secondary of said single transformer and a plurality of step down transformers connected in groups to the secondaries of said four transformers, and means for attenuating currents of predetermined frequency connected to the means coupling said last-mentioned antenna and said last-mentioned amplifiers.

2. In a system of the character described a plurality of antennas each responsive to currents of substantially different high frequencies, a plurality of high frequency amplifiers each responsive to dfferent bands of frequencies of the order of 2.5 megacycles wide and including one of said antenna responsive frequencies, means coupling said antennas and amplifiers which are responsive in similar bands, an antenna responsive to currents of the broadcast frequency range, a plurality of amplifiers each of which are responsive to different bandswithin said range of broadcast frequencies, means coupling said last-mentioned antenna and said last-mentioned amplifiers, circuits resonant to the frequency range of each of said first and each of said second mentioned amplifiers connected to the output of said amplifiers, a distributing network effectively connected to said resonant circuits, said distributing network including a single two to one step down transformer and four transformers having a two to one step down ratio connected to the secondary of said single transformer and a plurality of step down transformers connected in groups to the secondaries of said four transformers, and means for attenuating currents of predetermined frequency connected to the means coupling said last-mentioned antenna and said last-mentioned amplifiers.

WENDELL L. CARISON. VERNON D. LANDON. 

